In the fabrication of a semiconductor device or semiconductor material, a semiconductor wafer is processed in an enclosure defining a reaction chamber at a relatively high temperature above 1000° C., with the wafer being placed adjacent to or in contact with a resistive heater coupled to a power source. For a cylindrical heater, the wafer can be placed on a support and the support heated by the heater. In this process, the temperature of the semiconductor wafer is held substantially constant and uniform, varying in the range of about 1° C. to 10° C.
U.S. Pat. No. 5,343,022 discloses a heating unit for use in a semiconductor wafer processing process, comprising a heating element of pyrolytic graphite (“PG”) superimposed on a pyrolytic boron nitride base. The graphite layer is machined into a spiral or serpentine configuration defining the area to be heated, with two ends connected to a source of external power. The entire heating assembly is then coated with a pyrolytic boron nitride (“pBN”) layer. U.S. Pat. No. 6,410,172 discloses a heating element, wafer carrier, or electrostatic chuck comprising a PG element mounted on a pBN substrate, with the entire assembly being subsequently CVD coated with an outer coating of AlN to protect the assembly from chemical attacks.
Although graphite is a refractory material that is economical and temperature resistant, graphite is corroded by some of the wafer processing chemical environments, and it is prone to particle and dust generation. Due to the discontinuous surface of a conventionally machined graphite heater, the power density varies dramatically across the area to be heated. Moreover, a graphite body, particularly after machining into a serpentine geometry, is fragile and its mechanical integrity is poor. Accordingly, even with a relatively large cross sectional thickness, e.g., above about 0.1 inches as typical for semiconductor graphite heater applications, the heater is still extremely weak and must be handled with care. Furthermore, a graphite heater changes dimension over time due to annealing which induces bowing or misalignment, resulting in an electrical short circuit. It is also conventional in semiconductor wafer processing to deposit a film on the semiconductor which may be electrically conductive. Such films may deposit as fugitive coatings on the heater, which can contribute to an electrical short circuit, a change in electrical properties, or induce additional bowing and distortion.
One approach to improving the stability of graphite heaters is to coat the graphite body with a coating layer of a nitride such as boron nitride and a protective overcoat layer typically of the same material as the coating layer. Generally, the graphite body is machined into a desired shape or configuration defining a heating path with heating elements. The path can be, for example, a contiguous path having a space or gaps between adjacent heating elements. To provide a structure with sufficient support for handling and coating, the graphite body is machined to leave graphite bridges between the heating elements. The protective coating, e.g., pyrolytic boron nitride, is applied to the graphite body. Application of the coating layer to the graphite body produces a connecting layer consisting of the coating layer material overlying the graphite bridges. The coated heater body is then machined to remove the graphite bridges from the structure, which would cause the heater to short circuit if left in place. This requires machining through the coating, which leaves areas of exposed graphite. The heater may be machined to leave the connecting layer formed from the coating material.
While this connecting layer may provide support to the graphite heater, the heater must be coated again to coat the areas of exposed graphite. This design might still exhibit high stress from coefficient of thermal expansion (CTE) mismatch stress (between the graphite and boron nitride material) and thermal stress at elevated operating temperatures. High stress can result in early failure in the heating device.